What is Christian Prayer?

According to John Piper, "Prayer is the essential activity of waiting for God - acknowledging our helplessness and His power, calling upon Him for help, seeking His counsel...Prayer is the antidote for the disease of self-confidence, which opposes God's goal of getting glory by working for those who wait for Him" (Desiring God, pg. 170-171).

Prayer glorifies God by our recognition that we need Him to act and work for us. And, prayer is a filling up of our joy in God. These twin truths are seen by comparing John 14:13 and 16:24

Whatever you ask in my name, this I will do, that the Father may be glorified in the Son. (John 14:13) 

Until now you have asked nothing in my name. Ask, and you will receive, that your joy may be full. (John 16:24). 

The reason prayer brings joy is that it is fellowship with our beloved God, and because through His response to our prayers we are equipped for serving Him, which brings us delight.